This invention relates to the field of low dropout voltage regulators and, more particularly, to a noise reduction architecture of low dropout voltage regulators for substantially reducing noise.
Voltage regulators play a critical role in the proper operation of a large number of modern electronic circuits. It would be virtually impossible to operate the numerous electronic devices such as, for example, PCs or cell phones in the absence of integrated circuit low dropout voltage regulators.
Low dropout voltage regulators produce a regulated output voltage even when the unregulated input voltage from a power source falls to a level very close to that of the regulated output voltage. Because battery voltages typically decrease as batteries are discharged battery operated electronic devices commonly employ low dropout voltage regulators, but their use is by no means limited thereto.
A conventional voltage regulator comprises an input port and an output port, a pass transistor, which is the path element controlled by an error amplifier. A first non-inverting input of the error amplifier is connected to a reference voltage and another inverting input is coupled to a node within a voltage divider coupling the output port to ground. The amplifier compares the reference voltage with a feedback voltage developed at the node and amplifies the difference. Therefore, the gate voltage is controlled by the amplifier based upon the difference between the feedback voltage developed at the node and the reference voltage.
Numerous embodiments of low dropout voltage regulators addressing various application problems are disclosed in the prior art, for example, in U.S. Pat. Nos. 5,539,603, 5,552,697, 5,563,501, 5,686,821, 6,144,250, 6,188,212 and, 6,198,266, which are incorporated hereby for reference.
With increasing clock speed of modern electronic circuits, the modem electronic circuits are becoming more susceptible to high frequency noise inhibiting proper operation of the same. In order to reduce noise some prior art systems employ a low pass filter interposed between the reference voltage and the inverting input of the error amplifier. However, this solution does not address the problem of noise produced by the large resistance of the internal resistor voltage divider.
It is, therefore, an object of the invention to provide a low dropout voltage regulator comprising a noise reduction architecture, which is capable of substantially reducing the noise produced by the internal resistor voltage divider.
It is further an object of the invention to provide a low dropout voltage regulator which is capable of driving a wide range of loads.
According to the invention there is provided a low dropout voltage regulator comprising:
an input port for receiving an input voltage signal;
an output port for providing a regulated output voltage signal;
a bandgap reference interposed between the input port and ground for providing a reference voltage signal;
a voltage divider comprising a first and a second resistor interposed between the input port and ground;
an error amplifier having a first input terminal coupled to the band gap reference, an output terminal coupled to a first node interposed between the voltage divider and the input port and a second input terminal coupled to a second node interposed between the first and the second resistor of the voltage divider, the error amplifier for comparing a bandgap reference signal received at the first input terminal with a feedback signal received at the second input terminal and for providing a first output voltage signal in dependence thereupon;
a X1 buffer having a first input terminal coupled to the output terminal of the error amplifier, a second input terminal coupled to the output port and an output terminal coupled to a gate terminal of a pass transistor, the pass transistor having a source terminal connected to the input port and a drain terminal connected to the output port, the X1 buffer for providing gain to the first output voltage signal in order to drive a load connected to the output port; and,
a low pass filter interposed between the first node and the X1 buffer for filtering the first output voltage signal.
According to the invention there is further provided a low dropout voltage regulator comprising:
an input port for receiving an input voltage signal;
an output port for providing a regulated output voltage signal;
a bandgap reference interposed between the input port and ground for providing a reference voltage signal;
a voltage divider comprising a first and a second resistor interposed between the input port and ground;
a comparison stage having a first input terminal coupled to the band gap reference, an output terminal coupled to a first node interposed between the voltage divider and the input port and a second input terminal coupled to a second node interposed between the first and the second resistor of the voltage divider, the comparison stage for comparing the reference voltage signal received at the first input terminal with a feedback signal received at the second input terminal and for providing a first output voltage signal in dependence thereupon;
a gain stage having an input terminal coupled to the output terminal of the comparison stage for receiving the first output voltage signal and an output terminal coupled to the output port for providing the regulated output voltage signal, the gain stage for providing gain to the first output voltage signal in order to drive a load connected to the output port; and,
a filter element interposed between the first node and the gain stage for filtering the first output voltage signal.
According to an aspect of the invention there is provided a method for providing a regulated output voltage signal comprising the steps of:
providing an input voltage signal;
providing a bandgap reference voltage signal;
using a comparison stage, comparing the bandgap reference voltage signal with a feedback signal, the feedback signal being in dependence upon an output voltage signal provided by the comparison stage and the input voltage signal; filtering the output voltage signal; and,
using a gain stage, providing gain to the output voltage signal and providing a regulated output voltage signal in dependence thereupon.
The noise reduction architecture of the low dropout voltage regulator according to the invention is highly advantageous by providing high efficiency while high frequency noise is substantially reduced.